This invention relates to improvements in load-lifting mast assemblies for lift trucks. More particularly the invention relates to highly extensible free lift masts, especially four-stage masts, which provide a high degree of forward visibility for the lift truck operator while the truck is traveling with the load carriage substantially lowered, substantially uniform speed of the load carriage throughout its range of travel and maximum load-carrying capacity.
Highly extensible lift truck mast assemblies, particularly those of the four-stage type, present difficult problems with respect to resolving various competing design objectives. These competing objectives include:
(1) high forward visibility; PA1 (2) substantially uniform speed of the load carriage throughout its range of travel; PA1 (3) "free lift", i.e. initial raising of the load without simultaneous mast extension; and PA1 (4) minimum extension of the load forwardly of the lift truck front axle to maximize load-carrying capacity.
In a number of prior four-stage masts, free lift is provided by utilizing two separately-actuatable hydraulic rams, one being mounted upon the inner mast section for raising the load carriage relative thereto during free lift, and the other ram being mounted on another mast section at a position behind the first ram for extending the mast. Such fore-and-aft placement of two separate hydraulic rams, however, tends to maximize the fore-and-aft dimension of the mast assembly, thereby maximizing the extension of the load carriage forwardly of the lift truck front axle and thereby diminishing the load-lifting capacity of the truck. Moreover it is virtually impossible, as a practical matter, to provide both rams with a low collapsed height (preferably half the height of the fully-contracted mast) necessary for a high degree of operator forward visibility with the load carriage in a substantially lowered transporting position, because of the inordinate number (four) of separate lift chain sets which would be required to provide both free lift and full mast extension with two separate rams of such low collapsed height.
One four-stage mast, similar to the three-stage mast shown in McIntosh U.S. Pat. No. RE 27,731, has previously been conceived wherein an outer mast section and an additional set of lifting chains for extending the mast sections are added to the three-stage structure shown in the patent. The resultant device, while avoiding a fore-and-aft dual ram arrangement, does not provide a ram assembly having a low collapsed height as is required for good operator forward visibility. Rather the collapsed ram assembly is the same height as the fully-contracted mast and therefore remains directly in the center of the operator's view when the load carriage is in the lowered or transporting position. Moreover load carriage speed, assuming a uniform volumetric flow of fluid into the ram assembly, must be substantially higher during mast extension than during free lift because the ram assembly relies on "hydraulic sequencing" to ensure that free lift precedes mast extension during elevation of the load. That is, mast extension must require a substantially higher hydraulic pressure than does free lift to ensure that free lift precedes mast extension.
Although it would be possible to improve the operator visibility of the four-stage mast described in the previous paragraph by substituting therein a ram assembly of the type shown in Quayle U.S. Pat. Nos. 3,172,339, 3,235,033 or 3,235,034 because such ram assembly, being of the three-stage type, collapses to a height approximately equal to two-thirds of the height of the fully-contracted mast, the problem of nonuniformity of load carriage speed would become substantially worse. This is due to the fact that the four-stage mast chain reeving provides only a two-to-one ratio of load carriage speed to ram extension speed during free lift while providing a three-to-one ratio during mast extension, coupled with the fact that the effective hydraulic working area of the Quayle type of ram assembly becomes smaller as the ram extends causing the ram to extend gradually faster assuming a uniform volumetric flow of hydraulic fluid into the ram assembly. Thus load carriage speed would be more than 50% higher during mast extension than during free lift assuming a uniform volumetric supply of hydraulic fluid, such increase in speed being unacceptable from a viewpoint of operator control and safety during lifting operations.
The achievement of all of the above-listed competing objectives in a mast having only three stages is exemplified by the structure shown in the Farmer et al U.S. Pat. No. 4,008,648, the disclosure of which is hereby incorporated herein by reference. Here a single ram assembly having a low collapsed height (reaching only to approximately the longitudinal midpoint of the fully-contracted mast) provides both free lift and mast extension. Uniform speed of the load carriage throughout its entire range of travel is ensured by chain reeving which provides a ratio of two-to-one between carriage speed and ram extension speed during both free lift and mast extension, and by an internal ram structure which provides for constant ram extension speed when pressurized fluid is introduced thereto at a substantially constant rate. The constant-speed ram approach taught by this patent to resolve the aforementioned objectives cannot, however, be applied to a four-stage mast and would, in fact, be counter-productive with respect to the objective of uniform load carriage speed because the mast extension chains of a four-stage mast, for optimum design, would provide a substantially higher ratio of carriage speed to ram extension speed during mast extension than during free lift, in contrast to a comparable three-stage construction where the ratios are the same.
Other problems encountered in the construction of four-stage masts relate to the arrangement of four high-strength mast sections in transverse, side-by-side relationship, together with associated multiple sets of lift chains and hydraulic conduits, without requiring inordinate space in both the transverse and fore-and-aft directions so as to maximize visibility and minimize extension of the load forwardly of the lift truck front axle. In prior three-stage masts such objectives have been accomplished by means of staggered mast nesting arrangments such as those shown in British Pat. No. 1,182,962 and Ohta U.S. Pat. No. 3,638,761. Moreover, an advantageous chain arrangement for three-stage masts, also for aiding operator visibility, is shown in the aforementioned McIntosh U.S. Pat. No. RE 27,731. With four-stage masts however, the problem of minimizing the required space is substantially more severe due to the presence of one additional mast section, at least one additional set of lifting chains and additional hydraulic conduits necessary to accommodate the four-stage arrangement.